This invention relates to optical components generally, that is, components which operate in the optical domain. Such components are designed for use in various systems which utilize optical fibers, such as optical communications systems, optical data processing systems and optical switching systems. Specifically, it relates to an improved optical memory cell.
The development of optical fiber and optical semiconductor technologies in recent years has made possible various types of optical communications systems and optical switching systems. To utilize the full bandwidth and speed available in optical systems, it has become necessary to design and fabricate optical logic and switching components which eliminate the necessity for electrical-to-optical and optical-to-electrical conversions.
Optical memory cells are indispensable to time-division and self-routing optical switching systems. Currently, optical fiber delay lines are commonly used as memory cells in such systems, and are described in the literature. These delay lines are fixed in length and hence are not useful for switching systems which require flexible delays. A particular disadvantage is that the required physical length of such delay lines is usually very long. This causes the delay lines to be very bulky.
Recently a reentrant delay line memory, comprising a directional coupler and a multiple-loop delay line fiber, has been developed and demonstrated in an experimental photonic time-slot interchanger, R. A. Thompson and P. O. Giordano, "An experimental photonic time-slot interchanger using optical fibers as reentrant delay-line memories," J. Lightwave Technologies, vol. LT-5, pp. 154-162, Jan. 1987. In such a reentrant delay line memory, an input port and an output port of a directional coupler are connected by a delay line fiber to form a closed loop. The other input port and output port of the directional coupler are used for receiving and outputting the optical pulse, respectively. The directional coupler can be operated in the straight-through state or in the cross-over state. In the straight-through state, an optical packet in the delay line fiber will circulate around the closed loop formed by the delay line fiber and the directional coupler. The optical packet consists of optical pulses each representing a digital signal bit. In the cross-over state, the optical packet may enter the delay line fiber and/or exit from it. These conventional structures which circulate multiple-pulse optical packets are operationally disadvantageous. In particular, since the delay-line fiber is much longer than the pulse width of one optical pulse in order to accommodate the entire optical packet, the memory device requires a very precise timing control of the operational states of the directional coupler. Without this precision, an optical pulse may be split into two consecutive pulses during outputting, causing cross-talk between time slots. Furthermore, this precision is necessary to prevent destruction of the pulse sequence which occurs when individual pulses in a multi-bit optical packet are unintentionally interchanged during outputting, resulting in an output pulse train that is improperly sequenced relative to the input packet sequence.